Providing a phosphor layer, such as a yellow YAG phosphor, over a blue LED die is common. Typically, a phosphor is provided directly over a blue LED die to make white light. Blue light leaking through the phosphor, combined with the phosphor light, produces white light. There are many ways to deposit the phosphor layer over the LED die.
The blue light exciting the phosphor causes the phosphor to emit photons in all directions. For a relatively thin phosphor layer, it can be assumed that 50% of the phosphor light is emitted upward away from the LED die, and 50% of the phosphor light is emitted toward the LED die and impinges on the LED die surfaces. For gallium nitride (GaN) based LED dies, about 15% of the phosphor light impinging on the LED die surface is absorbed by the LED material and about 85% is reflected back towards the phosphor layer. Therefore, since about half of the phosphor light impinges on the LED surface, and 15% of that light is absorbed, about 7.5% of all light emitted by the phosphor layer is wasted by being absorbed by the LED material. This is the largest loss mechanism in the system.
What is needed is a technique to reduce this loss caused by the LED die's absorption of the phosphor light.